Electrical measuring instrument



March 27, 1934. J JOHNSON 1,952,626

Y ELECTRICAL MEASURING INSTRUMENT Original Filed Oct. 18, 1930 IN VENTOR.

MANFIEED J. JoH/vs orv,

By M, Qwww ATTORNEYS.

Patented Mar. 27, 1934 UNITED STATES PATENT OFFICE ELECTRICAL MEASURINGINSTRUMENT Manfred J. Johnson, Naugatuck, Conn., assignor to The LewisEngineering Company, Naugatuck, Conn.

Original application October 18, 1930, Serial No.

489,543. Divided and this application December 19, 1931, Serial No.582,040

Claims. (Cl. 73-32) This invention relates to electrical measuring theinstrument. This automatic compensation instruments and relates moreparticularly to an is achieved by having a magnetic shuntcompriselectrical measuring instrument adapted for use ing a piece ofsoft iron carried by the oi-metallic with a thermocouple for indicatingtemperatures. Spring, referred to in the preceding paragraph,

5 This application is a division of my co-pending and which, on movementof the spring under the 60 application, Serial No. 489,543, filedOctober 18, influence of temperature changes, bridges vari- 1930. ablythe gap between the poles of the galvanom- An object of the invention isto produce a thereter magnet to weaken the magnetic field actingmocouple and an associated galvanometer which on the galvanometer coilas the resistance of the form in themselves a complete and accuratetemelectrical conductors increase 65 perature measuring unit, requiringno auxiliary The invention will now be described with refsource ofelectrical current, and which is unerence to the drawing, of which:

afiected by external temperature changes. Fi 1 is a plan View Of aportion of a v Another object of the invention is to correct fornometer, according to this invention, showing external temperaturechanges at the cold juncthe bi-metallic spring attached to theindicating 70 tion of a thermocouple in an electrical measuring scale;

instrument associated with the thermocouple. Fig. 2 is an X-ray viewthrough the side of Another object of the invention is to correct agalvanometer, according to this invention,

for the effect of temperature changes on the reshowing the internalarrangement of the galvasistance of the electrical conductors of anelecnometer; 75 trical measuring instrument. Fig. 3 is a plan view of aportion of a galva- In one embodiment of the invention, an ordinometer,according to this invention, and shows nary moving coil type ofgalvanometer, having the arrangement of the magnetic shunt carriedcompensating means for temperature changes, is by the bi-metallicspring, relative the pole pieces 0 used to indicate temperature, and iselectrically of the galvanometer permanent magnet; 80 connected to athermocouple which is in intimate Fig. 4 is a schematic view of athermocouple, contact with the surface, the temperature of according tothis invention, electrically conwhich is to be measured. nected to agalvanometer;

According to one feature of the present inven- Fig. 5a is a view of athermocouple unit contion, a temperature measuring instrument capanectedbetween a pair of washers, one of which 85 ble of automatic compensationfor extreme temis clamped by a spark plug against the surface peraturechanges and requiring no auxiliary of an internal combustion engine;source of electrical current is provided. Fig. 5b is an enlargedsectional view of the According to another feature of the invention,washers of Fig. 5a showing the thermocouple the thermocouple element isconnected directly contained therebetween; 90 to the moving coil of thegalvanometer so that Fig. 5c is a top view of the lower washer of thecold junction, of the thermocouple, has always Fig. 5-?) showing thearrangement of the thermothe same temperature as that of thegalvanometer couple unit in a groove in the washer; elements. Fig. 6a isa side view .of a clamp, for housing According to another feature of theinvention, the hot junction of a thermocouple, which is 95 temperaturechanges at the cold junction of the adapted to be clamped onto thecooling fin of an thermocouple are compensated for by automatic internalcombustion engine; and

adjustment of the scale of the galvanometer to Fig. 6b is a sectionalview along the lines indicate temperature. This automatic compensa-6b-6b of Fig. 6a.

tion is achieved through the use of a bi-metallic Referring now to Fig.4 of the drawing, the m0 coil spring, which rotates about its centerwith thermocouple element 10 extends continuously temperature changes,and to which is connected from the hot junction 11 all the way to thetermithe indicating scale of the galvanometer, which nal studs 12 of thegalvanometer. The conducis shifted by the rotation of the bi-metallicSpring tors 13, contained within the casing of the galvato change itsposition relative the pointer of the nometer and connecting the innerend of the m5 galvanometer as temperature changes occur. terminal studs12 to the galvanometer coil 14,

According to another feature of the invention, are of the same metal asthe thermocouple elethe efiect of temperature changes on the resistment.With this arrangement, it is seen that the ance of the electricalconductors of the galvanomcold junction of the thermocouple isefiectively eter are automatically compensated for within containedwithin the galvanometer casing and is no therefore at the sametemperature at all times as the moving coil 14 and the bi-metallicspring 15 of the galvanometer. There is practically no temperature dropin the large size terminal studs 12, due to their large size, highconductivity, and short length, so for all practical purposes the coldjunction of the thermocouple may be presumed to be within thegalvanometer casing.

Referring now to Figs. 1, 2, 3, and4, the temperature compensatingdevices will be explained in detail. The galvanometer shown by Fig. 2 isof the well known DArsonval type except for the temperature compensatingdevices added according to this invention. The moving coil 14 is mountedon the shaft 35, on which is also mounted the hair springs 36 and 37 andthe indicating needle 20. The permanent magnet 25 is so mounted in thecasing 38 of the instrument that its pole pieces 23 and 24 are arrangedon each side of the coil 14. The currents generated in the thermocouple10 (Fig. 4) pass through the terminal studs 12, the leads 13 which areof thermocouple metal, the hair springs 36 and 37, and through the coil14 which is rotated on the shaft by the interaction between its fieldand the field between the pole pieces 23 and 24 of the magnet. Thehair-springs 36 and 37 serve to control the movement of the coil 14 andto return it towards its zero position when the energizing currentsdecrease. The coiled bi-metal lic spring 15 has its inner end 17 fixedlyattached to the galvanometer casing. Its outer end 18 is connected bythe arm 19 to the indicating scale 16 of the galvanometer. Thebi-metallic spring may consist of invar metal on one side and brass onthe other, and such a spring, due to the difierences in theco-efiicients of expansion of the two metals of which it is made up,

as is' well known, responds to temperature changes by rotating about itscenter when its temperature changes. Since the indicating scale 16 isconnected to the free end of the bi-metallic coil 15, it is caused tomove and to shift its position relative the pointer 20 of thegalvanometer. Since the bi-metallic spring 15 is at the same temperatureas the cold junction of the thermocouple, the scale 16 is shiftedrelative the pointer 20 of the galvanometer, as the temperature of thecold junction of the thermocouple changes.

The accuracy of the galvanometer depends upon a the difierence intemperature existing between the cold junction and the hot junction ofthe thermocouple, and since, 'in an instrument of this kind, it is notpractical to maintain the cold junction of the thermocouple at aconstant temperature, the movement of the indicating scale, relative theindicating needle of the galvanometer, adequately compensates forchanges in cold junction temperatures. For example, as the cold junctiontemperature increases, there is less difference of temperature existingbetween the cold junction and the hot junction of the thermocouple sothat the needle of the galvanometer does not move over the indicatingscale as far as it should, but since the free end of the bi-metallicspring 15 moves, due to the increase of temperature at the coldjunction, the scale 16 is moved bodily past the indicating needle of themeter so that the needle indicates a larger reading-on the indicatingscale than it would were not the temperature changes compensated for.Likewise, when the cold junction temperature decreases so that a greaterdifierence in temperatue exists between the cold junction and hotjunction of the thermocouple so that the galvanometer needle indicatestoo large a reading, the bi-metallic spring rotates in the oppositedirection and moves the scale relative the pointer so that the correctsmaller reading is indicated. In all cases, the constants of thegalvanometer elements and of the thermocouple are so related that forevery change of cold junction temperature, the indicating scale shiftsan amount just sufiicient to compensate for the change of temperatureAnother error which enters into the accuracy of an electrical indicatinginstrument is the change in resistance of the copper windings as theirtemperature changes. For example, the copper conductors, making up themoving coil 14 of the galvanometer, increase in resistance as theirtemperature increases. Since an increase in resistance of the movingcoil will affect the accuracy of the instrument, due to the fact that aportion of the electrical current will be consumed in the coil inovercoming the increased resistance, the coil will not move to asufiicient degree to indicate the amount of actuating current and thecorresponding temperature to be measured. To enable the coil to move tothe same extent, as it would were not its resistance increased by theincrease of temperature, the magnetic force acting upon the coil isreduced a corresponding amount, enabling the coil to move to the sameposition that it would were not 'its temperature increased. This isaccomplished by having the bi-metallic spring 15 carry a soft magneticshunt 22 (see Figs. 2 and 3 of the drawing) so that as the bi-metalliccoil 15 rotates about its center with changes in temperature, themagnetic shunt 22 is moved to more completely bridge and to lesscompletely bridge respectively, the gap between the pole pieces 23 and24 of the permanent magnet 25 of the galvanometer. As the temperaturesurrounding the instrument increases, the resistance of the copper coil14 increases, so that were the magnetic force constant, the coil wouldnot move to as great a degree as it should, but due to the increase intemperature, the free end of the bimetallic spring 15 moves and placesthe magnetic shunt 22 further from the pole piece 24 of the magnet 25,thus less completely bridging the gap between the pole pieces of themagnet and strengthening the magnetic field acting upon the moving coil14. Likewise, a decrease in temperature, causing a decrease in theresistance of the coil 14, tends to cause the coil to indicate a largerreading on the scale, and is compensated for by the movement of theshunt 22 towards the pole piece 24, thus weakening the magnetic fieldacting on the coil and resisting the tendency of the coil to cause thepointer 20 to over-read. The constants of the arrangement are so chosenthat changes in resistance of the co l 14 are exactly compensated for bycomparative changes in the magnetic flux acting upon the coil.

In mounting the hot junctions of thermocouples in a metallic surface,the temperature of which it is desired to measure, it has been thepractice heretofore to drilltwo small holes in the surface, to insertthe two thermocouple wires, and to pein over the edges of the holes toclamp the wires. This method is open to the very serious objection thatconsiderable heat is conducted away from the hot junction by thethermocouple wires, this resulting in the instrument indicating too lowtemperatures. Another objection is that it is difficult to mount thethermocouple in such a fashion, and the thermocouple wires break veryeasily. According to this invention, the thermo- 150 couple is placedwithin an enclosure maintained at the temperature of the surface, thetemperature of which it is desired to measure, in such a way that aconsiderable portion of the thermocouple is heated to the sametemperature, thus preventing conduction of the heat away from thehot-junction itself. One way in which this may be done may be seen withreference to Figs. 5a, 5b, and 5c. The two washers 26 and 2'7 haveannular grooves cut. as shown, in their contacting sides, leaving anopening of circular cross section between the two washers in which aconsiderable portion of the thermocouple i0 is inserted, as shown. Thewashers 26 and 27 are of such a size that they are adapted to be mountedbetween the base of a spark plug and the spark plug mounting. This isshown clearly in Fig. 5a of the drawing. A considerable portion of thethermocouple is inserted between the washers, which is heated to thesame temperature as the hot junction so that no appreciable conductionof heat from the heated junction can occur.

Figs. 6a and 6b indicate another method of mounting a thermocouple,according to this invention. The mounting there shown comprises the twoplates 28 and 29 which are clamped by means of the bolt 30 on each sideof a cooling fin 31 of an engine. The lower portions of the clamps 28and 29 are ribbed to insure intimate contact with the cooling fin 31. Atthe extreme lower ends of the plates 28 and 29 are drilled the holes 32and 33 respectively, in which are mounted, as shown by Fig. 6a, thethermocouple, which in this figure is designated as 34. The thermocouple34, being in intimate contact for a considerable length on both sides ofthe cooling fin 31is heated to the temperature of the coolingfin for aconsiderable distance away from the hot junction so that no appreciableconduction from the heated junction can occur.

The temperature compensating devices described are so simple and may bemade so compactly that very small galvanometers may be used for accuraterecording. The galvanometers used may be of the standard type which areeasily modified to include the temperature compensating features, andwhich are easily calibrated. The complete temperature measuringinstrument, made in accordance with this invention, is capable ofmeasuring a wide range of temperatures, rendering such an instrumentpeculiarly fitted for airplanes, automobiles, and for other outdoorapplications where wide ranges of temperatures are i expected.

Whereas, one thermocouple element has been shown in Fig. 4 as actuatingits associated galvanometer, it should be understood that two or morethermocouple elements can be connected in series relation where it isdesired to measure extremely high temperatures.

Whereas, one embodiment of the invention has been described for thepurpose of illustration; it should be understood that the invention isnot limited to the precise details described, but can be modified inmany ways without departing from the spirit of the invention.

What is claimed is:

1. A temperature measuring instrument, comprising in combination, agalvanometer, and a thermocouple having a hot junction and a coldjunction, the hot junction being arranged at the source of heat, thetemperature of which is to be measured, and the cold junction beingarranged within the casing of said galvanometer, said galvanometercomprising a magnetic field,

a movable coil arranged within said field and responsive to electricalcurrents from said then mocouple, an indicating needle movable with saidcoil, an indicating scale adjacent said needle, 9. bi-metallic springwithin the casing of said galvanometer, and at the same temperature asthe cold junction of said thermocouple, which, when temperature changesoccur at said galvanometer, acts to vary means which in turn varies themagnetic field acting upon said coil to compensate for changes in theresistance thereof, said spring acting to shift said indicating scalerelative said needle to compensate for the eifeet of temperature changesat the cold junction of said thermocouple.

2. A temperature measuring instrument comprising, in combination, agalvanometer and a thermocouple, the elements of the galvanometer beingmaintained at substantially the temperature of the cold junction of thethermocouple, a magnet within said galvanometer, a coil in the field ofsaid magnet movable in response to elec trical currents generated insaid thermocouple,

an indicator movable with said coil, a scale adjacent said indicator, asingle means in said galvanometer movable with temperature changesthereat, means movable by said means for compensating for the effect ofvariations of temperature of the cold junction of said thermocouple byshifting the position of said indicator and said scale relative eachother, and means movable by said first mentioned means for compensatingfor the effect of temperature changes on the resistance of theelectrical conductors in said galvanometer by varying the magnetic forceacting on said coil.

3. A temperature measuring instrument comprising, in combination, a.galvanometer and a thermocouple, the elements of the galvanometer beingmaintained at substantially the temperature of the cold junction of thethermocouple, a magnet within said galvanometer, a coil in the field ofsaid'magnet movable in response to electrical currents generated in saidthermocouple,

an indicator movable with said coil, a scale adtemperature of the coldjunction of said thermocouple by shifting the position of said indicatorwith said scale relative each other, and means movable by said elementfor compensating for the effect of temperature changes on the resistanceof the electrical conductors in said gal- 3g vanometer by varying themagnetic force acting on said coil.

4. A temperature measuring instrument comprising, in combination, agalvanometer and a thermocouple, the elements of the galvanometer 5being maintained at substantially the temperature of the cold junctionof the thermocouple, a magnet within said galvanometer, a coil in thefield of said magnet movable in response to electrical currentsgenerated in said thermocouple,

an indicator movable with said coil, 2. scale adjacent said indicator, acoiled spring in said galvanometer which expands with temperaturechanges thereat, means moved by said coiled spring for compensating forthe effect of variations of temperature of the cold junction of saidthermocouple by shifting the position of said indicator and said scalerelative each other, and means moved by said spring for compensating forthe eflect of temperature changes.on the revanometer expansible withtemperature changes thereat, means movable by said coiled spring forcompensating for the effect of variations of temperature of the coldjunction of said thermocouple by shifting the position of said scale,and means also movable by said spring for compensating for the effect oftemperature changes on the resistance of the electrical conductors insaid galvancmeter by shortening the gap between the pole pieces of saidmagnet for varying the magnetic field acting on said coil.

MANFRED J. JOHNSON.

